Nonlinear resistor and nonlinear resistor composition

ABSTRACT

A RESISTOR COMPOSITION HAVING A NONLINEAR VOLTAGE CHARACTERISTIC CONSISTING ESSENTIALLY OF ZINC OXIDE AND, AS AN ADDITIVE, AT LEAST ONE MEMBER TAKEN FROM THE GROUP CONSISTING OF LEAD FLUORIDE, BARIUM FLUORIDE OR STRONTIUM FLUORIDE, AND A NONLINEAR RESISTOR MADE FROM SAID COMPOSITION. THE NONLINEAR RESISTOR COMPOSITION AND THE RESISTOR MADE THEREFROM HAVE THE ELECTRICAL PROPERTIES THEREOF FURTHER IMPROVED BY THE ADDITION OF AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF COBALT FLUORIDE, MANGANESE FLUORIDE, STANNOUS FLUORIDE, NICKEL FLUORIDE, CHROMIUM FLUORIDE, BISMUTH OXIDE, COBALT OXIDE, AND MANGANESE OXIDE.   D R A W I N G

April 25, 1972 KESHI MASUYAMA ET AL NONLINEAR RESISTOR AND NONLINEAR RESISTOR COMPOSITION Filed July 24, 1970 yes/Ha 1m INVENTORS BY QMM $5M ATTORNEYS United States Patent Office 3,658,725 NONLINEAR RESISTOR AND NONLINEAR RESISTOR COMPOSITION Takeshi Masuyama, Takatsuki, Michio Matsuoka, Hirakata, and Yoshio Iida, Fujishirodai, Japan, assiguors to Matsushita Electric Industrial Co., Ltd., Kadoma,

Osaka, Japan Filed July 24, 1970, Ser. No. 57,976 Int. Cl. H01b 1/06 US. Cl. 252-518 12 Claims ABSTRACT OF THE DISCLOSURE A resistor composition having a nonlinear voltage characteristic consisting essentially of zinc oxide and, as an additive, at least one member taken from the group consisting of lead fluoride, barium fluoride or strontium fluoride, and a nonlinear resistor made from said composition. The nonlinear resistor composition and the resistor made therefrom have the electrical properties thereof further improved by the addition of at least one member selected from the group consisting of cobalt fluoride, manganese fluoride, stannous fluoride, nickel fluoride, chromium fluoride, bismuth oxide, cobalt oxide, and manganese oxide.

V n 4?) (1) Where V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation:

n: lo 2/ 1) where V and V are the voltage at given currents I and I respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the extent to which the resistors depart from ohmic characteristics.

In conventional varistors comprising germanium or silicon p-n junction diodes, it is difficult to control the C-value over a wide range because the nonlinear voltage property of these varistors is not attributable to the bulk of the resistor material but to the p-n junction between the body of the varistor and an electrode or electrodes. On the other hand, silicon carbide varistors have nonlinear voltage properties due to the contacts among the 3,658,725 Patented Apr. 25, 1972 2 individual grains of silicon carbide bonded together by a ceramic binding material, and the C-value is controlled by changing the dimension in the direction in which the current flows through the varistors. Silicon carbide varistors, however, have a relatively low n-value and are prepared by firing in a non-oxidizing atmosphere, especially for the purpose of obtaining a lower C-value.

An object of the present invention is to provide a resistance composition for a resistor having a nonlinear voltage characteristic, i.e. non-ohmic properties, due to the bulk thereof and having a controllable C-value.

Another object of the present invention is to provide a composition for a resistor having a nonlinear voltage characteristic characterized by a high n-value.

Another object of the present invention is to provide a resistor made from such a composition.

The details of the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the single figure is a partly cross-sectional view of a resistor having a nonlinear voltage characteristic according to the invention.

Before proceeding with a detailed description of the resistance composition and resistors contemplated by the invention, the resistor construction will be described with reference to the aforesaid figure of drawing wherein reference character 10 designates, as a whole, a resistor having a nonlinear voltage characteristic comprising, as its active element, a sintered body having a pair of electrodes 2 and 3 applied to opposite surfaces thereof. Said sintered body 1 is prepared in a manner hereinafter set forth and can be in any form such as circular, square or rectangular plate form. Wire leads 5 and 6 are attached conductively to the electrodes 2 and 3, respectively, by a connection means 4 such as solder or the like.

A resistor having a nonlinear voltage characteristic according to the invention comprises a sintered body of a composition consisting essentially of, as a major part, 90.0 to 99.95 mole percent of zinc oxide and, as an additive, 0.05 to 10.0 mole percent of at least one member selected from the group consisting of lead fluoride, barium fluoride and strontium fluoride. Such a resistor has non-ohmic resistance due to the bulk itself. Therefore, its C-value can be changed without impairing the n-value by changing the distance between the opposite surfaces of a body thereof. A shorter distance results in a lower C-value.

A higher n-value can be obtained when said additive consists essentially of at least one member selected from the group consisting of 0.5 to 2.0 mole percent of lead fluoride, 0.5 to 2.0 mole percent of barium fluoride and 0.3 to 3.0 mole percent of strontium fluoride in accordance with the invention.

The n-value is further elevated when said sintered body further includes 0.05 to 10.0 mole percent of at least one member selected from the group consisting of cobalt fluoride, manganese fluoride, stannous fluoride, nickel fluoride and chromium fluoride.

According to the present invention, the stability with respect to ambient temperature and during an electric load life test can be improved when said additive consists essentially of at least one member selected from the group consisting of 0.5 to 2.0 mole percent of lead fluoride, 0.5 to 2.0 mole percent of barium fluoride and 0.3 to 3.0 mole percent of strontium fluoride and 0.1 to 3.0 mole percent of at least one member selected from the group consisting of cobalt tfluoride, manganese fluoride, stannous fluoride, nickel fluoride and chromium fluoride.

The n-value is greatly elevated when said additive consists essentially of 0.05 to 5.0 mole percent of bismuth oxide and at least one member selected from the group consisting of 0.5 to 2.0 mole percent of lead fluoride, 0.5 to 2.0 mole percent of barium fluoride and 0.3 to 3.0 mole percent of strontium fluoride.

An extremely high n-value can be obtained when said additive consists essentially of 0.05 to 5.0 mole percent of cobalt oxide and at least one member selected from the group consisting of 0.5 to 2.0 mole percent of lead fluoride, 0.5 to 2.0 mole percent of barium fluoride and 0.3 to 3.0 mole percent of strontium fluoride. Further, according to the present invention, an extremely high n-value can be obtained also when said additive consists essentially of 0.05 to 5.0 mole percent of manganese oxide and at least one member selected from the group consisting of 0.5 to 2.0 mole percent of lead fluoride, 0.5 to 2.0 mole percent of barious fluoride and 0.3 to 3.0 mole percent of strontium fluoride.

The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials of the compositions described in the foregoing description are mixed in a wet mill so as to produce homogeneous mixtures. The mixtures are dried and pressed in a mold into desired shapes at a pressure of from 100 kg./cm. to 1000 kg./cm. The pressed bodies are sintered in air at a given temperature for 1 to 3 hours, and then furnace-cooled to room temperature (about to about 30 C.).

The sintering temperature is determined by the desired electrical resistivity, non-linearity and stability and ranges from 1000 to 1450 C.

The mixtures can be preliminarily calcined at about 700 C. and pulverized for easy fabrication in the subsequent pressing step. The mixture to be pressed can be admixed with a suitable binder such as water, polyvinyl alcohol, etc.

It is advantageous that the sintered body have the opposite surfaces lapped by abrasive powder such as silicon carbide having a particle size of 300 mesh to 1500 mesh.

The sintered bodies are provided, on the opposite surfaces thereof, with electrodes by any available and suitable method such as an electroplating method, a vacuum evaporation method, a metallizing method or a spraying or silver painting method.

The nonlinear properties are not affected to any practical extent by the kinds of electrodes used, but are affected by the thickness of the sintered bodies. Particularly, the C-value varies in proportion to the thickness of the sintered bodies, while the n-value is almost independent of the thickness. This means that the nonlinear voltage property is due to the bulk of the body, and not to the electrodes.

Lead wires can be attached to the electrodes in a per se conventional manner by using conventional solder having a low melting point. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent in order to connect the lead wires to the electrodes.

Resistors having a nonlinear 'voltage characteristic according to this invention have a high stability with respect to temperature and in a load life test, which is carried out at 70 C. at a rating power for 500 hours. The n-value and C-value do not change very much after heating cycles and a load life test. It is advan ageous for achievement of a high stability with respect to humidity that the resultant voltage variable resistors be embedded in a humidity-proof resin such as epoxy resin or phenol resin in a per se well known manner.

Presently preferred illustrative embodiments of the invention are as follows.

EXAMPLE 1 A mixture of zinc oxide and an additive in amounts as shown in Table 1 are mixed in a wet mill for 3 hours. The

mixture is dried and then calcined at 700 C. for 1 hour. The calcined mixture is pulverized by a motor-driven ceramic mortar for 30 minutes and then pressed in a, mold into a shape 17.5 mm. in diameter and 2.5 mm. thick at a pressure of 500 kg./cm.

The pressed body is sintered in air at 1350 C. for 1 hour, and then furnace-cooled to room temperature (about 15 to about 30 (3.). The sintered disc has the opposite surfaces lapped by silicon carbide having a particle size of 600 mesh. The resulting sintered disc is 14 mm. in diameter and 1.5 mm. thick. Silver paint electrodes commercially available are attached to the opposite surfaces of the sintered disc by painting. Then lead wires are attached to the silver electrodes by soldering. The electric characteristics of the resultant resistors are shown in Table 1. It will be readily understood that the zinc oxide sintered body having incorporated therein lead fluoride, barium fluoride, or strontium fluoride in an amount of 0.05 to 10.0 mole percent is useful for a resistor having a nonlinear voltage characteristic, and particularly the addition of 0.5 to 2.0 mole percent of lead fluoride, 0.5 to 2.0 mole percent of barium fluoride or 0.3 to 3.0 mole percent of strontium fluoride makes the nonlinear voltage property more excellent.

Similar results can be obtained from a zinc oxide sintered body having incorporated therein two or more fluorides selected from the group consisting of lead fluoride, barium fluoride and strontium fluoride. For example, in a zinc oxide body prepared as described above to which 0.5 mole percent of lead fluoride and 0.5 mole percent barium fluoride have been added results in 200 volts C-value at 1 ma. and an n-value of 7.2.

TAB LE 1 Electrical characteristics Additive (mole C (at 1 ma.)

percent) EXAMPLE 2 Starting materials composed of 99.5 mole percent of zinc oxide and 0.5 mole percent of an additive as listed in Table 2 are mixed, dried, calcined and pulverized in the same manner as those of Example 1. The pulverized mixture is pressed in a mold into discs 17.5 mm. in diameter and 5 mm. thick at a pressure of 500 kg./cm.

The pressed bodies are sintered in air at 1350 C. for 1 hour, and then furnace-cooled to room temperature. The sintered discs have the opposite surfaces thereof ground by silicon carbide having a particle size of 600 until the discs have thicknesses as shown in Table 2. The ground discs are provided with electrodes and lead wires on the opposite surfaces in a manner similar to that of Example 1. The electric characteristics of the resultant resistors are shown in Table 2; the C-values vary approximately in proportion to the thicknesses of the sintered discs while the n-value is essentially independent of the thickness. It will be readily realized that the nonlinear voltage properties of the resistors are attributable to the sintered body itself.

3. A resistor as claimed in claim 1, wherein said additive is at least one member selected from the group consisting of 0.5 to 2.0 mole percent of lead fluoride, 0.5 to 2.0 mole percent of barium fluoride and 0.3 to 3.0 mole percent of strontium fluoride.

4. A resistor as claimed in claim 3, wherein said sintered body further includes 0.1 to 3.0 mole percent of at least one member selected from the group consisting of cobalt fluoride, manganese fluoride, stannous fluoride, nickel fluoride and chromium fluoride.

5. A resistor as claimed in claim 3, wherein said sintered body further includes 0.05 to 5.0 mole percent of bismuth oxide.

6. A resistor as claimed in claim 3, wherein said sintered body further includes 0.05 to 5.0 mole percent of cobalt oxide.

7. A resistor as claimed in claim 3, wherein said sintered body further includes 0.05 to 5.0 mole percent of manganese oxide.

8. A resistor composition having a nonlinear voltage characteristic, said composition consisting essentially of, as a major part, 99.95 to 90.0 mole percent of zinc oxide and, as an additive, 0.05 to 10.0 mole percent of at least one member selected from the group consisting of lead fluoride, barium fluoride and strontium fluoride.

9. A resistor composition as claimed in claim 8, further including 0.05 to 10.0 mole percent of at least one member 10 selected from the group consisting of cobalt fluoride, manganese fluoride, stannous fluoride, nickel fluoride and chromium fluoride.

10. A resistor composition as claimed in claim 8, Wherein said additive is at least one member selected from the group consisting of 0.5 to 2.0 mole percent of lead fluoride, 0.5 to 2.0 mole percent of barium fluoride and 0.3 to 3.0 mole percent of strontium fluoride.

11. A resistor composition as claimed in claim 10, further including 0.1 to 3.0 mole percent of at least one member selected from the group consisting of cobalt fluoride, manganese fluoride, stannous fluoride, nickel fluoride and chromium fluoride.

12. A resistor composition as claimed in claim 10, further including 0.05 to 5.0 mole percent of bismuth oxide.

References Cited UNITED STATES PATENTS 3,551,356 12/1970 Bowman 252-520 3,503,029 3/ 1970 Matsuoka 252-5l8 3,089,856 5/1963 Cyr et a1 252-518 DOUGLAS J. DRUMMOND, Primary Examiner US. Cl. X.R. 252519, 521 

